A power supply receives AC power and converts it to steady DC power for computer operation. The general electric power users occasionally encounter a blackout or brownout condition. Hence the present power supply usually provides protection function to guard against abnormal voltage. The abnormal conditions of AC power used in the general houses mainly can be divided into two types: one is drop out in which the voltage drops significantly and instantly from a normal value. The duration could be very short, but the voltage is excessively low or at a lower level constantly, thus the power becomes unusable. This could be caused by damage of an electric equipment or abrupt start of a greater load at the user end (such as a motor of a great capacity being started suddenly). Another type is brown out in which the voltage slowing drops below the normal voltage, and output of electric equipments also decreases until stops (such as a lamp gradually dims until fully turn-off). In the event that the foregoing two types of abnormal conditions occur, the power supply has to generate a corresponding delay and shutdown time series to protect the computer and power supply it self. R.O.C. patent publication No. 501830 entitled “Improved exchanged power supply” provides a judgment capability for abnormal voltage. It sets a delay time period for excessive low voltage (4 seconds was set in that patent). In the event that the excessive low voltage condition lasts not longer than 4 seconds, the delay unit increases the ON duration time of a switch Q11 of a power factor correction unit to keep the power supply in normal operation. And the power factor correction circuit has a capacitor C53 at an output end that maintains a constant voltage. Hence output can be maintained intact. But lengthening the ON duration time of the switch could result in overheat and burn-out of the switch Q11. Hence in the excessive low voltage condition the circuit could be damaged before the delay time of 4 seconds has elapsed. The object of protection cannot be fully accomplished. Moreover, the patent mentioned above does not take into account the condition of drop out in a very short time. As the drop out condition takes place very quickly, the technique of the previous patent cannot respond fast enough, and a voltage bouncing effect could happen on the output. This could cause damage to the computer in operation. As the conventional technique previously discussed does not provide satisfactory solution to overcome the drop out and brown out problems, there are still rooms for improvement.